1. Field of the Invention
The invention relates to the field of binders for glass fibers. Specifically, the invention relates to a low emissions process for making a co-binder including phenol/formaldehyde binder and polyacrylic acid binder, and the co-binder composition made by the process.
2. Background Information
Manufacture of glass fiber thermal insulation may be carried out by a continuous process whereby molten glass flows from a melting furnace, divides into streams, and attenuates into fibers. The fibers are collected on a conveyor belt to form a mat. The fibers are bonded together to form an integral structure by a curable liquid resinous binder sprayed onto the fibers as they are dropping onto the collecting conveyor belt. The binder-coated layer of fiber is then passed through a curing oven to cure the binder. Under the curing conditions, any remaining aqueous solvent is evaporated, and the phenol/formaldehyde resole cures to a thermoset state. The fibers in the resulting fiberglass product are thus partially coated with a thin layer of thermoset resin, which tends to accumulate at the junctions where fibers cross each other. The resulting product exhibits higher recovery than a fiberglass product not incorporating a binder. The cured binder imparts strength and resiliency to the structure.
The organic binders most commonly employed are heat-curable aqueous thermosetting resins of the phenol formaldehyde type. Generally, the binder system requirements are for a low-cost water soluble or water dispersable composition which can be easily applied and readily cured. The product should be relatively stable for periods of time long enough to permit mixing and application at temperatures ordinarily encountered in fiberizing plants, and the composition should be dilutable. The cured binder product must provide a strong bond with sufficient elasticity and thickness recovery to permit reasonable deformation of the thermal insulating product. Thickness recovery is important if full insulating value is to be obtained after installation.
Manufacturers of glass fiber products also find they must deal with the formaldehyde vapors associated with handling ordinary resoles which pose a number of environmental and safety issues.
Typically, when a phenol-formaldehyde resole resin is used as a binder, e.g. for manufactured boards and fiber insulation products, the binder releases a significant amount of formaldehyde into the environment during cure. Formaldehyde also can be released subsequently from the cured resin, particularly when the cured resin is exposed to acidic environments. Such formaldehyde release is undesirable, particularly in enclosed spaces. Formaldehyde may be inhaled by workers and comes into contact with the eyes, the mouth, and other parts of the body. Formaldehyde is malodorous and is thought to contribute to human and animal illness. It is, therefore, desirable to reduce the release of formaldehyde into the environment.
Various techniques have been used to reduce formaldehyde emission from formaldehyde-based resins. In particular, various formaldehyde scavengers (i.e., chemicals usually added to the resin during or after its preparation) have been used in an attempt to reduce formaldehyde emissions. Urea is often selected because it is less expensive compared to other formaldehyde scavengers. Urea acts as a formaldehyde scavenger both at, and subsequent to, the manufacture of the product. Urea is typically added directly to the phenol-formaldehyde resin, to produce a urea-extended phenol-formaldehyde resole resin (known in the art as "premix" or "pre-react). The resin can be further treated or applied as a coating or binder, as desired, and then cured. The urea addition also contributes to improved anti-punk characteristics for the cured binder. Urea is also used to extend phenol/formaldehyde resins for use in fiberglass binders. Urea is available at approximately 20% of the cost of the alkaline phenol/formaldehyde resoles commonly used in fiberglass binders. Thus, an extension of the binder with 30% urea provides a substantial cost savings. Accordingly, urea serves the dual function of providing a lower cost resin as well as reducing emissions of formaldehyde.
To obtain a typical urea-extended resole binder resin, a mixture of phenol and formaldehyde is reacted, with a suitable alkaline catalyst, in one or more steps. The resole resins are generally prepared by reacting a phenol with an excess molar proportion of formaldehyde in the presence of a basic catalyst, such as an inorganic alkaline catalyst or an amine catalyst. Most resins for the fiberglass industry are catalyzed with inorganic catalysts because of their low cost and non-volatility.
A typical phenolic resin to be used as a binder for glass fiber insulation is made at a formaldehyde/phenol mole ratio as high as six to virtually eliminate free phenol in the resin. The high formaldehyde/phenol ratio required to achieve the very low free phenol concentration results in high free formaldehyde concentrations. The high percentage of free formaldehyde in the resin must be scavenged by the addition of a large amount of urea or other formaldehyde scavenger. Urea is added after neutralizing the resin and most often just prior to use of the resin. When the urea is added, the level of free formaldehyde is typically reduced to about 0.5 to about 1.5% after the premix is allowed to react at room temperature for a few hours.
The reaction conditions, temperature, catalyst amount, etc., are adjusted to favor phenol methylolation reactions over condensation reactions. Notwithstanding the desire to favor methylolation reactions, some condensation of methylolated phenolic monomers still occurs, producing undesirable dimer species. Such dimers can form an undesirable crystalline precipitate during resin storage, especially in solutions with a low formaldehyde content. This precipitation phenomenon is a particular problem in processes using formaldehyde scavengers. The most common organic compounds that precipitate in urea-extended resole resins are a two unit methylolated phenolic species (tetradimer) and a dimethylolated urea (DMU). Crystallization of tetradimers can cause the blockage of lines, interruption of normal operations, and reductions in resin use efficiency. The crystallized material is difficult to dissolve and hinders uniform application of the resin to the glass fiber. Due to the very poor tetradimer stability of the premix solutions of inorganic base-catalyzed resins and urea, vigorous precautions must be taken with the inorganic base-catalyzed resins, to avoid tetradimer crystal growth, for example, by regular cleaning of the storage tanks and lines, and by shortening the time between the preparation and use of the premix solution.
In the glass fiber industry, a phenolic resin is sometimes produced by a resin manufacturer, and then is sold to a glass fiber producer. Often urea cannot be added to the phenolic resin in the resin manufacturers site because the premix is not stable enough to permit it to be stored for two to three weeks without tetradimer precipitation. Consequently, most phenolic fiberglass resins are sold without any added urea.
Although efforts in the industry to eliminate or substantially reduce formaldehyde are well known, less well known is the fact that ammonia emissions are also under extreme scrutiny, with several states having exceptionally stringent requirements in this regard. Thus, it is desirable to lower both the formaldehyde and ammonia emissions from fiberglass binder compositions.
Accordingly, a need remains for a low emissions process for making a resinous binder low in free phenol and dimer content suitable for use in glass fiber based insulation mats. Such a need is fulfilled by the invention described in detail below.